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Case Reports

Abstract

Background: Spitz melanocytic tumors harboring RET kinase fusion are rare and poorly characterized. Recent studies indicate they can range from benign to malignant, and have relatively heterogeneous histopathology, which may be more determined by their 5’ fusion partners.   

Case:  A 10-year-old boy presented with a pigmented lesion on the back. Histologic sections demonstrated a well-circumscribed and roughly symmetric compound melanocytic proliferation with associated compact hyperkeratosis, lateral acanthosis and central epidermal attenuation. Junctional melanocytes were arranged in small nests and single cells with variable vertical orientation, focal peripheral clefting, and mild lentiginous spread.  Kamino bodies were noted.  Dermal melanocytes were arranged in small but tightly packed nests, which coalesced into a large central nodule.  Maturation was limited with dermal descent, with brisk background chronic inflammation and melanoderma.  Melanocytes showed moderate amounts of eosinophilic to variably vacuolated and pigmented cytoplasm.  Nuclei were round to ovoid, with minimal enlargement, and with open chromatin and prominent nucleoli. Rare dermal mitotic figures were noted.  Given the centrally nodular growth pattern and somewhat variable cytomorphology, NGS was performed.  A pathogenic fusion between MYH9 and RET genes at exon 31 and exon 12 respectively was identified. Tumor mutational burden was low (4.8 mutations/Mb), and  TERT promoter mutation/overexpression was negative. 

Discussion:  The current case represents only the second reported atypical Spitz tumor with MYH9::RET.  The current case adds to the collective knowledge regarding the histopathology and molecular features of RET fused Spitz tumors.